The present invention relates to epoxy compositions; in particular the present invention relates to stable low temperature curing epoxy-based compositions.
U.S. Pat. No. 4,225,691 (Crivello) describes a low temperature curable organic resin composition. The composition comprises a cationically polymerisable organic resin and a catalyst which has three components: a diaryliodonium salt, a copper salt and a reducing agent. Diaryliodonium salts are known as photoinitiators for cationic polymerisations. However Crivello, in his paper published in Journal of Polymer Science: Polymer Chemistry, Vol. 21, 97-109 (1983), reports that diaryliodonium salts undergo efficient thermal decomposition in the presence of copper(II) compounds. The ""691 patent (U.S. Pat. No. 4,225,691) discloses that compositions comprising a diaryliodonium salt employed with a catalyst comprising a mixture of a copper salt and a reducing agent can effect the cure of a cationically polymerisable organic material, such as an epoxy resin, spontaneously, or within a few minutes, without the use of external heat and under atmospheric conditions. The compositions must be stored in two parts for storage stability. Such compositions are known to those skilled in the art as two-part compositions.
U.S. Pat. No. 4,238,587 (also to Crivello) describes curable compositions comprising (i) a cationically polymerizable organic material, (ii) a diaryliodonium salt and (iii) a copper chelate or a mixture of a copper chelate and a reducing agent. The diaryliodonium salts are described as being effective thermal initiators for the polymerisation of cationically polymerisable materials such as epoxides, cyclic ethers and phenol-formaldehyde resins.
The compositions described in the ""691 and ""587 patents are suited for storage as two-part compositions due to the almost immediate cure of compositions containing the two parts mixed together. Combining the two parts of Crivello result in compositions which are very unstable at room temperature.
Developments to this technology have been proposed; for example WO 98/33645 (Ghosal) describes an epoxy resin composition containing particulate filler, a base resin comprising a polyepoxy resin, a cycloaliphatic epoxy-functional siloxane, an iodonium salt, a copper compound and optionally a toughener. The compositions are described as addressing the poor shelf life, high modulus, and slow cure of prior art epoxy resin compositions and are said to be useful as die-attach adhesives, polymer bumps, underfill encapsulants, and glob-top encapsulants.
U.S. Pat. No. 4,275,190 (Dudgeon) describes stabilising epoxy resin compositions catalyzed with combinations of diaryliodonium salts of complex halogenides and copper salt catalyst stabilized at room temperature by including an effective amount of a carboxyl-terminated copolymer of butadiene and acrylonitrile, or a hydroxy-terminated copolymer of butadiene and acrylonitrile or mixtures thereof. The compositions are described as being fully activatable at elevated temperatures.
It is desirable to provide a stable, low to moderate temperature curing, epoxy-based composition, which is suitable for formulation as a one-part composition.
Surprisingly it has been found that a curable one-part epoxy-based composition comprising;
(a) a polymerisable cycloaliphatic epoxy compound; and
(b) a diaryliodonium salt; and
(c) a salt or complex of copper, is stable at room temperature, as compared to the prior art compositions of Crivello which are unstable and cure rapidly at room temperature. The compositions comprising (a), (b) and (c) above are curable by exposure to temperatures of about 70xc2x0 C. to about 100xc2x0 C. such as 80xc2x0 C. to about 100xc2x0 C. The composition is suitable for formulation as a one-part composition.
The compositions of the invention are useful in particular chip bonding and in bonding of electronic components generally and in general bonding and marking applications described in more detail below.
Furthermore it has also surprisingly been discovered that addition of a component (d) an aromatic diketone shows improved stability as compared to a composition of components (a) to (c).
An alternative (or additional) stabiliser component (e) a nitroxyl free radical stabiliser may be added to a composition of components (a) to (c) above (and optionally component (d)) to stabilise the composition. Component (e) provides additional stability.
A component (f) a xcex2-diketone may also be used (as an additional or alternative stabiliser to component (d) and/or component (e)) to stabilise the composition.
The composition may have added one or more of components (d), (e) and (f).
It has also been discovered by the present inventors that a composition comprising components (a) to (c) and at least one of components (d), (e) or (f) may additionally comprise component (g) an xcex1-hydroxy ketone. Previously one-part epoxy compositions containing an xcex1-hydroxy ketone were found to be curable with rapid cure at room temperature. The compositions of the present invention are storage stable and are heat activatable at moderate temperatures such as those in the range of about 70xc2x0 C. to about 100xc2x0 C. such as about 80xc2x0 C. to about 100xc2x0 C.
It has been found that compositions comprising components (a) to (c) and in addition one or more of components (d), (e), (f) and (g), and in particular compositions comprising each of components (a) to (g), have highly desirable propertiesxe2x80x94being storage stable and yet remain curable at temperatures in the range of about 80xc2x0 C. to about 100xc2x0 C.
The compositions given above are suitable for formulation as one-part compositions, in the presence or absence or air.
A convenient and expedient method of curing compositions of the present invention is to expose the composition to heated air so as to effect cure.
A further component (h) a dye may be added to any of the compositions described above without adversely affecting the stability of the composition. The dye can be used to import a desired colour to the composition.